Human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS), encodes three enzymes, including the well-characterized proteinase belonging to the aspartic proteinase family, the HIV protease. Inhibition of this enzyme has been regarded as a promising approach for treating AIDS. Hydroxyethylamine isosteres have been extensively utilized in the synthesis of potent and selective HIV protease inhibitors. However, this modern generation of HIV protease inhibitors has created an interesting challenge for the synthetic organic chemist. Advanced x-ray structural analysis has allowed for the design of molecules that fit closely into active sites on enzymes creating very effective drug molecules. Unfortunately, these molecules, designed by molecular shape, are often difficult to produce using conventional chemistry.
The modem generation of HIV inhibitors has structural similarities in a central three-carbon piece containing two chiral carbons that link two larger groups on each side (see, e.g., Parkes, et al., J. Org. Chem., 59:3656-3664 (1994). Numerous synthetic routes to these isosteres have been developed. As illustrated below, a common strategy to prepare the linking group starts with an amino acid, such as phenylalanine, to set the chirality of the first carbon. Then, the linking group is completed by a series of reactions including a one-carbon homologization during which the old amino acid carbon is transformed into a hydroxy-functionalized carbon having the correct chirality. However, the commercial production of isosteres by this method presents serious challenges, generally requiring low-temperature organometallic reactions (Ghosh, et al., J. Org. Chem., 62:6080-6082 (1997) or the use of exotic reagents. 
A second approach, which is illustrated below, is to convert the amino acid to an aldehyde and to add the carbon by use of a Wittig reaction to give an olefin (see, Luly, et al., J. Org. Chem., 52:1487-1492 (1987). The olefin is then epoxidized. Alternatively, the aldehyde can be reacted with nitromethane, cyanide (see, Shibata, et al., Chem. Pharm. Bull., 46(4):733-735 (1998) or carbene sources (see, Liu, et al., Org. Proc. Res. Dev., 1:45-54 (1997). Instability and difficulty in preparation of the aldehyde make these routes undesirable (see, Beaulieu, et al., J. Org. Chem., 62:3440-3448 (1997). 
Other routes that have been published, but not commercialized are illustrated in FIG. 1.
One of the best reagents that can be used to add a single carbon to amino acids is diazomethane because it gives high yields and few side-products. In addition, diazomethane reactions are very clean, generating only nitrogen as a by-product. HIV inhibitor molecules need high purity because of the high daily doses required. As such, diazomethane is an ideal reagent for making high purity compounds. In spite of the documented hazards of diazomethane, processes have recently been developed that permit the commercial scale use of diazomethane to convert amino acids to the homologous chloromethyl ketones (see, U.S. Pat. No. 5,817,778, which issued to Archibald, et al. on Oct. 6, 1998; and U.S. Pat. No. 5,854,405, which issued to Archibald, et al. on Dec. 29, 1998). FIG. 2 illustrates examples of HIV protease inhibitors wherein the central linking group can be synthesized by the commercial use of diazomethane. FIG. 3 illustrates a general reaction scheme that can be used to prepare the S,S-epoxide compound using diazomethane.
The most useful amino acid isosteres are based on phenylanaline. The key intermediate in the synthesis of Sequinivir(copyright) (Roche) and Aprenavir(copyright) (Glaxo Wellcome) is the (S,S-)N-t-butoxycarbonyl-1,2-epoxy-4-phenyl-3-butanamine. Several other protease inhibitors, such as those described in Chen, et al. (J. Med. Chem., 39:1991-2007 (1996) or those under development (e.g., BMS-234475 or BMS-232623), use the diastereomeric (R,S-) N-t-butoxycarbonyl-1,2-epoxy-4-phenyl-3-butanamine.
Beginning with readily available (L)-phenylanaline, one is able to manufacture N-t-butoxycarbonyl-1-chloro-2-keto-4-phenylbutanamine (called xe2x80x9cchloroketonexe2x80x9d or xe2x80x9cCMKxe2x80x9d) using the methods described in the literature (see, e.g., Parkes, et al., J. Org. Chem., 59:3656-3664 (1994); Shaw, Methods in Enzymology, 11:677-686 (1967); and Dufour, et al., J. Chem. Soc. Perkin Trans. I, 1895-1899 (1986), the teachings of which are incorporated herein by reference). However, what are needed in the art are methods that allow one to produce reliably and in high-yields either diastereomer, i.e., the S,S or the R,S, from the common chloroketone starting material (see, FIG. 4). Quite surprisingly, the present invention fulfills this and other needs.
The present invention provides compounds and methods that can be used to convert the intermmediate halomethyl ketones (HMKs), e.g., chloromethyl ketones, to the corresponding S,S- and R,S-diastereomers. It is these chiral centers that determine the chiral centers in the HIV protease inhibitor and, thus, the efficacy of the drug. As explained herein, the present invention provides (1) reduction methods; (2) inversion methods; and (3) methods for preparing alkenes that, in turn, can undergo epoxidation reactions to form the desired R,S-epoxide. Using the various methods of the present invention, the R,S-epoxide and the intermediary compounds can be prepared reliably, in high yields and in high purity.
As such, in one embodiment, the present invention provides a method for selectively preparing an R,S-halomethyl alcohol (R,S-HMA) compound having the following general formula: 
the method comprising: reducing a compound having the following general formula: 
with a non-chelating, bulky reducing agent to form the R,S-HMA compound. In the above formulae, R1 is an amino acid side chain (e.g., a benzyl group, an S-phenyl group, an alkyl group and a para-nitrobenzene group, etc.); R2 is a blocking or protecting group (e.g., Boc, Cbz, Moc, etc.); and X1 is a leaving group (e.g., a halo group, such as chloro). In a presently preferred embodiment, the non-chelating, bulky reducing agent is a member selected from the group consisting of lithium aluminum t-butoxyhydride (LATBH), sodium tris-t-butoxyborohydride (STBH). In another presently preferred embodiment, the reduction is carried out in a solvent such as diethyl ether. Once formed, the R,S-HMA can be reacted with an alkali metal base to form an R,S-epoxide.
In another aspect, the present invention provides inversion methods that can be used to selectively prepare the R,S-epoxide. In one embodiment of the inversion method, R,S-epoxide is prepared by a four step process. More particularly, in one embodiment of the inversion method, the present invention provides a method for preparing an R,S-epoxide having the following general formula: 
the method comprising: (a) reducing a halomethyl ketone (HMK) compound having the following general formula: 
with a reducing agent to form an S,S-halomethyl alcohol (S,S-HMA) compound having the following general formula: 
(b) contacting the S,S-HMA compound of Formula II with a member selected from the group consisting of arylsulfonyl halides and alkylsulfonyl halides in the presence of an amine to form an S,S-halomethyl sulfonyl (S,S-HMS) compound having the following general formula: 
(c) contacting the S,S-HMS compound of Formula III with an acetate in the presence of a phase transfer catalyst and water to form an R,S-halomethyl acetate (R,S-HMAc) compound having the following general formula: 
and (d) contacting the R,S-HMAc compound of Formula IV with an alkali metal base to form the R,S-epoxide. In the above formulae, R1 is an amino acid side chain (e.g., a benzyl group, an S-phenyl group, an alkyl group, a para-nitrobenzene group, etc.); R2 is a blocking or protecting group; X1 is a leaving group (i.e., a halo group, such as chloro); R3 is a functional group including, but not limited to, arylsulfonyls and alkylsulfonyls (e.g., a mesyl group, a tosyl group, a triflate group, a nosyl group, etc.); and R4 is an acyl group derived from the acetate (e.g., an acetyl group).
In another embodiment of the inversion method, the present invention provides a method for preparing an R,S-epoxide compound having the following general formula: 
the method comprising: (a) contacting an S,S-halomethyl sulfonyl (S,S-HMS) compound having the following general formula: 
with a carbamate-forming acetate to form a cyclic carbamate; and (b) contacting the cyclic carbamate with an alkali metal base to form the R,S-epoxide. In the above formulae, R1, R2, R3 and X1 are as defined above. In a presently preferred embodiment, the carbamate-forming acetate is sodium trichloroacetate.
In yet another aspect, the present invention provides a method for preparing R,S-epoxide by the epoxidation of an alkene. More particularly, the present invention provides a method for preparing an alkene having the following general formula: 
the method comprising: (a) contacting a compound having the following general formula: 
with a hydrohalo acid to form a compound having the following general formula: 
(b) reducing a compound of Formula II with a reducing agent to form a compound having the following general formula: 
and (c) dehalohydroxylating a compound of Formula III to form the alkene. In the above formulae, R1, R2, and X1 are as defined above. Once prepared, the alkene can be converted to the R,S-epoxide using, for example, m-chloroperbenzoic acid.
Other features, objects and advantages of the invention and its preferred embodiments will become apparent from the detailed description which follows.